[cmake] don't REQUIRE externals deps in root, add boost_headers (#265)

boost_headers is actually required for some distros apparently (notably
Arch, thanks username227 for spotting it), forces the bundled package
for it as well

AUR cpp-httplib also does not work with this for some reason, so in
order to support `CMAKE_DISABLE_FIND_PACKAGE_httplib` (required for
Arch if `cpp-httplib` is installed there), removes REQUIRED from the
external propagation calls (wasn't needed anyway idk why I added it)

I really wish we had more Arch developers tbh

Signed-off-by: crueter <crueter@eden-emu.dev>

Reviewed-on: #265
Reviewed-by: Lizzie <lizzie@eden-emu.dev>

externals/CMakeLists.txt

@@ -317,6 +317,17 @@ if (YUZU_USE_EXTERNAL_VULKAN_SPIRV_TOOLS)
     )
 endif()
 
+# Boost headers
+if (NOT Boost_ADDED AND NOT TARGET Boost::headers)
+    AddPackage(
+        NAME boost_headers
+        REPO "boostorg/headers"
+        SHA 0456900fad
+        HASH 50cd75dcdfc5f082225cdace058f47b4fb114a47585f7aee1d22236a910a80b667186254c214fa2fcebac67ae6d37ba4b6e695e1faea8affd6fd42a03cf996e3
+        BUNDLED_PACKAGE ON
+    )
+endif()
+
 # TZDB (Time Zone Database)
 add_subdirectory(nx_tzdb)